An organic electroluminescent element (hereinafter, also referred to as organic EL element) is a light-emitting element that has a structure in which a light-emitting layer containing a luminescent compound is sandwiched between a cathode and an anode and that utilizes emission of light (fluorescence•phosphorescence) caused by deactivation of excitons generated by recombination of holes injected from the anode and electrons injected from the cathode by the application of an electric field within the light-emitting layer. Further, an organic EL element is an all-solid-state element that includes organic material films with a thickness of only about a submicron level provided between electrodes, and can emit light at a voltage of about several volts to several tens of volts, and is therefore expected to be used for next-generation flat-panel displays and lighting.
Princeton University has developed an organic EL element for practical use and has reported an organic EL element using phosphorescence emission from an excited triplet state (see, for example, Non-Patent Literature 1), and since then, materials that emit phosphorescence at room temperature have been actively studied (see, for example, Patent Literature 1 and Non-Patent Literature 2).
Further, an organic EL element utilizing phosphorescence emission can achieve, in principle, luminous efficiency about 4 times higher than that of a conventional organic EL element utilizing fluorescence emission, and therefore development of materials thereof as well as research and development of layer structures and electrodes of light-emitting elements has been performed all over the world. For example, many compounds, mainly, heavy metal complexes such as iridium complexes have been synthesized and studied (see, for example, Non-Patent Literature 3).
As described above, a phosphorescence emission system is a very high-potential system. However, an organic EL element utilizing phosphorescence emission is significantly different from an organic EL element utilizing fluorescence emission in that how to control the position of a luminescent center, especially, how stably light can be emitted by recombination within a light-emitting layer is an important technical issue to be solved to improve the efficiency and lifetime of the element.
Under the circumstances, a multi-layered element is well-known in recent years, which has a light-emitting layer, a hole transport layer provided adjacent to the anode side of the light-emitting layer, and an electron transport layer provided adjacent to the cathode side of the light-emitting layer (see, for example, Patent Literature 2). As the light-emitting layer, a mixed layer using a host compound and a phosphorescence-emitting compound as a dopant is often used.
On the other hand, from the viewpoint of material, materials that have high carrier transportability and are thermally or electrically stable are required. Particularly, a blue phosphorescent compound itself has high triplet excitation energy (T1), and therefore in order to utilize blue phosphorescence emission, development of applicable peripheral materials and precise control of a luminescent center are strongly required.
As a typical blue phosphorescence-emitting compound, FIrpic (Bis[2-4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)) is known, which achieves a shorter emission wavelength by fluorine substitution of phenylpyridine as a primary ligand and use of picolinic acid as an auxiliary ligand. Such a dopant is combined with carbazole derivatives or triarylsilanes as host compounds to achieve higher efficiency of elements, which however significantly deteriorates the emission lifetime of the elements. Therefore, such trade-off needs to be improved.
As a means for improving the trade-off, improvement in the thermal stability of a metal complex by caging has been considered. For example, there is a technique in which the generation of degradation products during vapor deposition is prevented by improving the thermal stability of a metal complex as a material for organic EL elements to improve the performance of the elements.
Further, metal complexes having a specific ligand have been found in recent years as high-potential blue phosphorescent compounds (see, for example, Patent Literatures 3 and 4).
Improvement in the performance of a phosphorescence-emitting material for phosphorescent organic EL elements influences the achievement of full-scale use of organic EL elements for lighting and electronic displays, and is therefore an issue of greatest concern in the field of organic EL materials, but development of a phosphorescent material having improved performance is a difficult issue.